Collisional radiative coarse-grain model for ionization in air

Abstract

We present a reduced kinetic mechanism for the modeling of the behavior of the electronic states of the atomic species in air mixtures. The model is built by lumping the electronically excited states of the atomic species and by performing Maxwell-Boltzmann averages of the rate constants describing the elementary kinetic processes of the individual states within each group. The necessary reaction rate coefficients are taken from the model compiled by Bultel et al. [“Collisional-radiative model in air for earth re-entry problems,” Phys. Plasmas 13, 043502 (2006)10.1063/1.2194827]. The reduced number of pseudo-states considered leads to a significant reduction of the computational cost, thus enabling the application of the state of the art collisional radiative models to bi-dimensional and three-dimensional problems. The internal states of the molecular species are assumed to be in equilibrium. The rotational energy mode is assumed to quickly equilibrate with the translational energy mode at the kinetic temperature of the heavy species as opposed to the electronic and the vibrational modes, assumed to be in Maxwell-Boltzmann equilibrium at a common temperature TV. In a first step we validate the model by using simple zero- and one-dimensional test cases for which the full kinetic mechanism can be run efficiently. Finally, the reduced kinetic model is used to analyze the strong non-equilibrium flow surrounding the FIRE II flight experiment during the early part of its re-entry trajectory. It is found that the reduced kinetic mechanism is capable of reproducing the ionizational non-equilibrium phenomena, responsible for the drastic reduction of the radiative heat loads on the space capsules during the re-entry phase.